This invention relates to the preparation of organic sulfur compounds. It further relates to a method of preparing mixed esters containing alkyl mercaptoalkanoates. It also relates to the preparation of compounds which are intermediates in the preparation of polymer sealant compositions.
The reaction of hydrogen sulfide with olefinically unsaturated organic compounds is an important process for the production of organic sulfur compounds which are intermediates in the production of compositions having a variety of commercial uses.
The reaction of H.sub.2 S with an olefinically unsaturated carboxylate in the presence of a basic catalyst is a known method of preparing alkyl mercaptoalkanoates along with varying amounts of other organic sulfur compounds including thiobis(alkylalkanoates) and dithiobis(alkylalkanoates). Methods for the preparation of these "mixed esters" from H.sub.2 S and an olefinically unsaturated organic compound are disclosed in U.S. Pat. Nos. 4,052,440, 4,067,901, and 4,113,707, among others.
These mercaptoalkanoates are intermediates in the preparation of polymers used to make elastomeric sealant compositions. To produce a polymer which displays the necessary physical properties after curing, it is desirable to produce a mixed ester product which is predominantly the mercaptoalkanoate. Although it is possible to remove the mercaptoalkanoate from the product mixture and then prepare the desired mixture separately, it is more economical and convenient to prepare mixed esters in the desired proportions in a single step.
A generally preferred method of preparing mixed esters for ultimate use in sealant compositions is to react H.sub.2 S with a compound such as an alkyl acrylate in the presence of a weak base such as ammonium hydroxide. It has been found that during commercial production of the mixed esters, particularly during cold weather and periods of high consumption, in-plant grade H.sub.2 S can absorb as much as 15 weight percent CO.sub.2. The presence of this contaminant in the reaction mixture makes it impossible, even by varying the proportions of other components of the reaction mixture, to produce ester mixtures containing the alkyl mercaptoalkanoate in the desired proportions.
It is thus an object of this invention to provide a method of preparing mixed esters containing alkyl mercaptoalkanoates in the desired high proportions from unsaturated carboxylates and CO.sub.2 -contaminated H.sub.2 S.
It is a further object of the invention to minimize the reaction steps in the preparation of alkyl mercaptoalkanoates used as intermediates in the preparation of polymer sealant compositions.